1. Field of the Invention
The present invention relates to a method for producing a semiconductor device, more particularly to a method for forming impurity doped regions in a semiconductor substrate by using the self-alignment method wherein aluminum gate electrodes are used as a masking material.
2. Description of the Prior Art
The self-alignment method has conventionally been applied to the formation of source and drain regions of field effect transistors (FET) in semiconductor substrates. Since no space is necessary for position alignment of photo masking material in the self-alignment method, self-alignment is a very effective technique to increase the degree of semiconductor device integration.
After gate electrodes are formed on gate insulating layers, heat treatment is carried out at 500.degree. C. to activate the source and drain regions using the gate electrodes as a masking material. Therefore, in the self-alignment method, it is necessary that the material of conventional gate electrodes have a high heat resistance. Thus, polycrystalline silicon, which has a high heat resistance, has usually been used for conventional gate electrodes. Even considerable doping of impurities into the polycrystalline, however, fails to reduce the electrical resistance of the doped polycrystalline to the level of aluminum. Thus, the switching speed of the semiconductor device is lowered.
In recent years, laser annealing of semiconductors has been carried out to activate the source and drain regions. While aluminum has poor heat resistance, it reflects the laser beam well. Therefore, it should be possible to use aluminum for the masking material to form the source and drain regions by the ion-implanting or doping method, then anneal the regions by laser irradiation so as to produce FET's having aluminum gate electrodes by the self-alignment process. As a result, the integration density and switching speed of semiconductor devices can be improved. Japanese Unexamined Patent Publication No. 55-102271 describes the use of aluminum as gate electrodes and annealing an ion-doped layer by laser beam irradiation using the aluminum gate electrode as a masking material. However, in the process described in Japanese Unexamined Patent Publication No. 55-102271, the laser beam irradiation has a detrimental effect on the aluminum gate electrodes. Namely, the irradiation energy of the laser beam on the aluminum is so strong that it damages the aluminum despite much of it being reflected by the aluminum. Irradiation of 2.2 joules of energy by a ruby laser beam, for example, will fuse and further disperse the aluminum of gate electrodes. Irradiation of even 0.31 joules of energy by a laser beam will produce a crack network at the surface of the aluminum gate electrodes. Such irradiation energy therefore damages the aluminum gate electrodes themselves.
On the other hand, while irradiation of a low amount of energy by a laser beam, for example, 0.146 joules will not damage the aluminum, it will also not activate the impurity doped regions enough to lower the sheet resistivity. Namely, a region into which arsenic ion (As.sup.+) was doped under conditions 5.times.10.sup.15 cm.sup.-2 and 100 KeV energy can be reduced to only a sheet resistivity of 100 .OMEGA./.quadrature. by an irradiation energy of 0.146 joules compared with a sheet resistivity of 50 .OMEGA./.quadrature. by an irradiation energy of 0.3 joule.
As explained above, a high irradiation energy of a laser beam on aluminum gate electrodes will damage the aluminum, and a low irradiation energy of a laser beam on aluminum gate electrodes will not be sufficient to activate the impurity doped region to lower the sheet resistivity.